9.9 Declarations
In general, functions must be declared before they are used. Stan supports forward declarations, which look like function definitions without bodies. For example,
real unit_normal_lpdf(real y);
declares a function named unit_normal_lpdf
that consumes a
single real-valued input and produces a real-valued output. A
function definition with a body simultaneously declares and defines
the named function, as in
real unit_normal_lpdf(real y) {
return -0.5 * square(y);
}
A user-defined Stan function may be declared and then later defined, or just defined without being declared. No other combination of declaration and definition is legal, so that, for instance, a function may not be declared more than once, nor may it be defined more than once. If there is a declaration, there must be a definition. These rules together ensure that all the declared functions are eventually defined.
Recursive Functions
Forward declarations allow the definition of self-recursive or mutually recursive functions. For instance, consider the following code to compute Fibonacci numbers.
int fib(int n);
int fib(int n) {
if (n < 2) return n;
else return fib(n-1) + fib(n-2);
}
Without the forward declaration in the first line, the body of the definition would not compile.